Conventional ceramic glow plugs have used a multi-layered construction. Such glow plugs are described in U.S. Pat. Nos. 4,742,209, 5,304,778 and 5,519,187. In general, the glow plugs have a ceramic heater with a conductive core enclosed by insulative and resistive ceramic layers, respectively. The layers are separately cast and fitted together. The resulting green body is sintered to form a unitary ceramic heater. During use, these glow plug experience cyclic heating and cooling, which results in high internal stresses at the interfacial junction between the ceramic layers, promoting eventual failure of the glow plugs.
The internal stresses of a layered glow plug are mainly the result of differences in the coefficients of thermal expansion between the differently composed layers. The different layers of the glow plug expand and contract at different rates. Further, residual stresses are the result of manufacture, particularly from uneven contraction in the cooling period which occurs below the plastic deformation state of the ceramic composition, and from non-uniform attachment between the layers.
Oxidation and corrosion of the outer resistive surface in the highly corrosive atmosphere in a diesel engine cylinder is also a problem. The ceramic heater tip becomes caked with a carbon coating during normal use which reduces the lifespan and effectiveness of the glow plug. The presence of carbon residue in the engine cylinder is also highly undesirable and may damage the engine head.